The Morgan Arboretum of McGill University, with its 245 ha of forest and interspersed field habitats, is home to nearly 50 species of butterflies. Over the past two years I have tried to document all species occurring within the Arboretum and made note of the date of their earliest appearance. Many of the butterflies observed are the “expected” species, such as the Question Mark, White Admiral and the Monarch.

Others, though, were much more exciting finds: the Banded and Acadian Hairstreaks, the Baltimore Checkerspot and the Silver Spotted Skipper to name a few. Of all the highlight species found over the past two years, one that truly stands out is the Hackberry Emperor (Asterocampa celtis).

Hackberry Emperor looking down from a high perch. Credit: Christopher Cloutier

Like many other butterfly species, the Emperor is specific to one type of host plant for its larvae. You guessed it: the Hackberry Tree (Celtis occidentalis). Although the Arboretum lies within the native range for this tree, it is one that is rarely encountered. It is found naturally on the outskirts of the property and nowhere near the main walking trails; that is, until about 10 years ago when the Arboretum planted several trees near the parking lots along the main road. The trees today are no taller than 4m but are growing rapidly. This represents nearly the entire habitat in which the Emperors were discovered back in 2010, and this is the tale of their unusual discovery.

Unlike most of the species which I have documented over the years, this one came as a report from a concerned visitor to the Arboretum. I remember this case vividly as it was quite unique. A visitor to the Arboretum came by the gatehouse to mention that they were seeing a large butterfly up close. In fact, the butterfly was landing on them with regularity every time they passed by a certain location. This was something I had to see for myself. Not knowing what to expect I followed the man to where he encountered this critter and sure enough we were standing right next to the Hackberry plantation. Within less than a minute a butterfly alighted on my shoulder, a species I had never encountered before. I quickly collected it with my aerial net and brought it back to my office for a closer look.

It didn’t take long to discover that this beautiful butterfly was indeed the Hackberry Emperor. After doing a little bit of research, I realized that this was not the first time that this species had been encountered at the Arboretum, but it was the first time in nearly half a decade. I decided to have a little photo shoot with the insect just to get some record shots. I then gave it a sip of grape juice and brought it back to where I first captured it.

I decided to have a closer look at the Hackberry trees scattered about on the grassy lawn. There were only five trees, not more than twice my height, and I quickly noticed why the butterflies were here. They were breeding. After searching the gall-riddled leaves of the Hackberries, I discovered several clusters of eggs as well as some recently hatched first instar larvae. Again, upon my arrival several adults were patrolling the area trying to frighten me away, or maybe trying to get a closer look at who I was. It didn’t seem to matter what colour clothing I was wearing, they just seemed interested in large silhouettes near their nursery.

Since this first discovery I have encountered Hackberry Emperors every summer since. They are typically active in mid-June and their activity time extends into July and August. Their dependence on a single tree species makes this butterfly quite interesting. Had we chosen to plant a different species of tree as a windbreak for the parking area, we may not have ever encountered this butterfly again. It seems now that we have made an ideal artificial breeding habitat for this beautiful insect, and hopefully they choose to use it year after year, that is, as long as they abide by our strict “no harassing other visitors” policy.